Key discovered to cold tolerance in corn

Longer
growing season, growth in colder regions possible

Demand
for corn -- the world's number one feed grain and a staple food for many -- is
outstripping supply, resulting in large price increases that are forecast to
continue over the next several years. If corn's intolerance of low temperatures
could be overcome, then the length of the growing season, and yield, could be
increased at present sites of cultivation and its range extended into colder
regions.

Drs.
Dafu Wang, Archie Portis, Steve Moose, and Steve Long in the Department of Crop
Sciences and the Institute of Genomic Biology at the University of Illinois may
have made a breakthrough on this front, as reported in the September issue of
the journal Plant Physiology.

Plants
can be divided into two groups based on their strategy for harvesting light
energy: C4 and C3. The C4 groups include many of the most agriculturally
productive plants known, such as corn, sorghum, and sugar cane. All other major
crops, including wheat and rice, are C3. C4 plants differ from C3 by the
addition of four extra chemical steps, making these plants more efficient in
converting sunlight energy into plant matter.

Until
recently, the higher productivity achieved by C4 species was thought to be
possible only in warm environments. So while wheat, a C3 plant, may be grown
into northern Sweden and Alberta, the C4 grain corn cannot. Even within the Corn
Belt and despite record yields, corn cannot be planted much before early May and
as such is unable to utilize the high sunlight of spring.

Recently
a wild C4 grass related to corn, Miscanthus x giganteus, has been found to be
exceptionally productive in cold climates. The Illinois researchers set about
trying to discover the basis of this difference, focusing on the four extra
chemical reactions that separate C4 from C3 plants.

Each
of these reactions is catalyzed by a protein or enzyme. The enzyme for one of
these steps, Pyruvate Phosphate Dikinase, or PPDK for short, is made up of two
parts. At low temperature these parts have been observed to fall apart,
differing from the other three C4 specific enzymes. The researchers examined the
DNA sequence of the gene coding for this enzyme in both plants, but could find
no difference, nor could they see any difference in the behavior of the enzyme
in the test tube. However, they noticed that when leaves of corn were placed in
the cold, PPDK slowly disappeared in parallel with the decline in the ability of
the leaf to take up carbon dioxide in photosynthesis. When Miscanthus leaves
were placed in the cold, they made more PPDK and as they did so, the leaf became
able to maintain photosynthesis in the cold conditions. Why?

The
researchers cloned the gene for PPDK from both corn and Miscanthus into a
bacterium, enabling the isolation of large quantities of this enzyme. The
researchers discovered that as the enzyme was concentrated, it became resistant
to the cold, thus the difference between the two plants was not the structure of
the protein components but rather the amount of protein present.

The
findings suggest that modifying corn to synthesize more PPDK during cold weather
could allow corn, like Miscanthus, to be cultivated in colder climates and be
productive for more months of the year in its current locations. The same
approach might even be used with sugar cane, which may be crossed with
Miscanthus, making improvement of cold-tolerance by breeding a
possibility.